Abqaiq was the first Saudi oil target since Al-Qaeda launched attacks aimed at toppling Saudi Arabia's pro-Western monarchy in 2003. The country's deputy interior minister, Prince Muhammad bin Nayef, on August 27 escaped with light injuries in the first known assault on a member of the Saudi royal family.

"Thursday night we received a call to tighten security measures and car inspection at all gates," one security guard said.

"Even Aramco employees undergo inspection. There's a lack of trust as militants disguised in Aramco's cars in 2006 attacked the facility," he added.

An Aramco spokesperson was not immediately available to comment.

Al-Qaeda leaders have repeatedly called for attacks on oil facilities and Western interests in the Persian Gulf to destabilize U.S.-allied rulers and harm the economies of Western countries by blocking the flow of oil.

"This [Abqaiq] is a sensitive industrial area...security measures are a must," an Aramco employee said.

Saudi government security forces foiled the attack on Abqaiq in 2006.

Most Saudi oil is exported from the gulf via the huge pumping and processing facility at Abqaiq in the mainly Shi'ite Eastern Province.

Abqaiq handles crude pumped from the giant Ghawar field and ships it off to terminals Ras Tanura -- the world's biggest offshore oil-loading facility -- and Juaymah. It also pumps westwards across the kingdom to Red Sea export terminals.

"When such things happen, police, national guards, Aramco impose stricter measures at facilities...so that means more safety," said a Saudi-based contractor who works with Aramco.